1. Field of the Invention
The present invention relates to noise reduction in radio receivers and the like, and in particular, a noise reduction system for reducing noise during intervals between intelligence-carrying signals.
2. Description of the Prior Art
In the area of radio communications, the limiting factor in recovering intelligence from a modulated radio frequency signal is the presence of noise. This noise occurs in a number of forms. Effective methods for reduction of noise are known for some types of noise. For example, sharp impulse noise, such as from ignition and other pulse-like sources, is commonly reduced by blanking for a short interval which would normally have a negligible effect on the intelligence of the modulated signal. However, thermal noise, which generally has a uniform frequency distribution tailored by the response curves of the receiving system, and atmospheric noise is not susceptible to this type of noise reduction scheme. Even when such noise is at a relatively low level, it causes fatigue of the listener and detracts from the overall intelligibility of the signal.
Peaks of noise occurring during the presence of intelligence signals can be reduced somewhat with clippers and limiters, but excessive clipping will distort the intelligence signals and thereby reduce the intelligibility thereof. A significant portion of the disturbing noise may well occur in short intervals during which no intelligence is being received. An example is between words, sentences and syllables or between pulses in Morse code CW systems. Thus, it is apparent that an improvement in the signal may be made without significantly affecting intelligibility by eliminating or greatly reducing the noise amplitude during such intervals.
In U.S. Pat. No. 3,973,210 to Holden, a system is described which reduces noise between signal envelopes by phase displacement between noise envelopes and the original composite signal envelopes at the input of an audio circuit. While this system has proved helpful for reception of CW or Morse code type signals, it is not suitable for reception of audio modulated signals because noise modulates the audio signal as well as being superimposed on it. Thus, there is a need for a system which will reduce the intersyllabic and word interval noise in a modulated audio signal to reduce fatigue of the listener and to increase the intelligibility of the audio signals.